NASA discovers first Earth-sized planets beyond our Solar System

View full sizeThis chart compares the first Earth-size planets found around a sun-like star to planets in our own solar system, Earth and Venus. NASA's Kepler mission discovered the new found planets, called Kepler-20e and Kepler-20f. Kepler-20e is slightly smaller than Venus with a radius .87 times that of Earth. Kepler-20f is a bit larger than Earth at 1.03 times the radius of Earth. Venus is very similar in size to Earth, with a radius of .95 times that our planet. (NASA image)

MOFFET FIELD, California - NASA's Kepler mission has discovered the first Earth-sized planets orbiting a sun outside our Solar System. NASA says the planets - Kepler-20e and Kepler-20f - are too close to their star to be in the so-called habitable zone. That's where where liquid water could exist on their surfaces.

But the two planets are the smallest so-called "exoplanets" ever confirmed around a star like the Earth's sun. The fact the Kepler mission could find them is considered very significant. "The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of a new study published in the journal Nature. "This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them."

Kepler-20e orbits its parent star every 6.1 days, NASA says, and Kepler-20f every 19.6 days. These short orbits mean very hot, inhospitable worlds. Kepler-20f is 800 degrees Fahrenheit, about like the planet Mercury. The surface temperature of Kepler-20e is more than 1,400 degrees Fahrenheit, hot enough to melt glass.

The Kepler space telescope detects planets and planet candidates by measuring dips in the brightness of stars as they pass in front, or transit, their tars. The Kepler science team requires at least three transits to verify a planet. To see a 46-image slide show of Kepler's work, including more images of what scientists believe the new planets look like, click this link.